As known, there is an increasing need to accurately dose small amounts of powdery or granulated drugs, thus limiting waste. A further need is dosing and filling the base of the capsule as rapidly as possible, in an automated way and without the operator coming into contact with the product.
The aforesaid needs are due to different and often concurrent causes, for instance the exorbitantly high cost of some drugs, the pharmaceutical tendency to the so-called “low dosage”, which reduces the amount of excipient, and the appearance of difficult-to-treat powders because of their fineness, their scarce flowability, their tendency to clot and/or because they must be extremely delicately treated from a mechanical point of view.
To this regard, the case of powdery inhalable medicines, which are difficult to dose because of their volatility and because their texture must not be altered during the dosing, for instance by means of mechanical stress, is highly revealing.
One of the most diffused methods involves the use of a sieve.
The equipments made according to this method are based on the known, everyday fact that, in a sieve filled with a powdery material, natural “bridges” tend to form in correspondence to the holes. By applying no stress, the sieve reaches a state of balance, wherein all holes are blocked and the material does not flow through them. By applying stress on the sieve, some of these bridges break down and the material flows through the holes until the surrounding conditions allow again said state of balance.
Both the amount of material passing through each hole of the sieve before the conditions for a new hole blockage occur and the percentage of holes opened up according to a certain kind of stress follow probabilistic laws. The dependence of the flow of material on the applied stress can be used for the dosage, but the regularity of this behaviour is generally directly proportional to the number of holes.
The relationship existing between the characteristics of the material and the size of the holes is even intuitively clear. In fact, with holes above a certain size the material passes completely through them without stress, whereas with holes below a certain size the powdery material does not pass at all, even if stress is applied.
Therefore, there is an optimal dosage size of the holes of the sieve, optimal size depending on the material and its conditions (e.g. granulometry, moisture, etc.). Furthermore, the size of the holes of the sieve is selected so that the material does not pass in the absence of vibrations, whereas it passes with the desired flow in the presence of stress. The adjustment of the size of the holes to the material is a critical point, since even modest variations in the material's characteristics can lead on the one hand to the blockage of the holes and on the other hand to mistakes due to an excessive flow.
The number of holes should be high to obtain a sufficiently regular behaviour and a larger flow in the starting phase for a quicker filling of the capsules. On the other hand, a huge number of holes, increasing the minimum amount of material released after the stress, lowers the resolution and therefore the accuracy in the achievement of the predetermined value.
The most evident problem of this technique is the clotting of the sieve, which compels a progressive increase of the intensity of stress until the amount of released material at the unblocking moment turns out to be excessive, thus involving a remarkable loss of time and a frequent rejection because of an erroneous dosage. However, also the obvious method of increasing the intensity of stress is often not enough to solve the problem; therefore, attempts have been made to adopt other solutions, such as an overturning of the container, the use of compound stresses or the insertion of decompacting devices.
Moreover, the situation is even more critical because of the clots present in the product to be dosed and, above all, because of the tendency of some kinds of material to form clots as a consequence of vibrations.
In brief, once defined certain operating conditions (kind of product, diameter and number of the holes, filling level of the sieve) the applied stress must be higher than a certain level to cause the unblocking and the subsequent fall of material. Once exceeded this threshold, the amount of falling material usually has a minimum value determining the obtainable resolution and therefore the chance that the final dosage falls within a certain tolerance range. The release of material highly depends on the number of applied stresses and less on their intensity.